A number of short peptides with significant activity as inhibitors of cell growth have been isolated from the Indian Ocean sea hare Dolabella auricularia (Bai, et al., Biochem. Pharmacology 40: 1859-1864 (1990); Beckwith et al., J. Natl. Cancer Inst. 85: 483-488 (1993) and references cited therein). These include Dolastatins 1-10 (U.S. Patent No. 4,816,444, issued to Pettit et al.) and Dolastatin-15 (European Patent Application No. 398558). Dolastatin 15, for example, markedly inhibits the growth of the National Cancer Institute's P388 lymphocytic leukemia (PS system) cell line, a strong predictor of efficacy against various types of human malignancies.
The exceedingly small amounts of the various Dolastatin peptides present in Dolabella auricularia (about 1 mg each per 100 kg sea hare) and the consequent difficulties in purifying amounts sufficient for evaluation and use, have motivated efforts toward the synthesis of these compounds (Roux et al., Tetrahedron 50: 5345-5360 (1994); Shioiri et al., Tetrahedron 49: 1913-24 (1993); Patino et al., Tetrahedron 48: 4115-4122 (1992) and references cited therein). Synthetic Dolastatin 15, however, suffers from drawbacks which include poor solubility in aqueous systems and the need for expensive starting materials for its synthesis. These, in turn, have led to the synthesis and evaluation of structurally modified Dolastatin 15 derivatives (see, for example, Bioorg. Med. Chem. Lett. 4: 1947-50 (1994); WO 93 03054; JP-A-06234790).
However, there is a need for synthetic compounds with the biological activity of Dolastatin 15 which have useful aqueous solubility and can be produced efficiently and economically.